The book of Deuteronomy played a central role in transformations that resulted in Mosaic Torah. It is commonly argued that the book results from a process of literary redactions, and it is the book in the hands of late 2nd Temple tradents, which made it central to later ideas of Jewish “book religion.” This article seeks to answer the following questions: When did Deuteronomy’s notion of book religion appear, by whom, and for what purpose? Central to these questions is the dating of the book’s remarks about writing down the Mosaic speech, which is the presupposition for the use of “Torah” as a comprehensive term. The article reassesses the different suggestions, then turning to the relation between oral proclamation and writing in the book, how the book legitimates oral teaching, and what it meant to the Jewish community in Persian-Hellenistic time to be accepted as Scripture. The final part addresses the question of the possible ideological and physical location of the authors.